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Coil: A Configuration Library.

Introduction
============
  Coil is a configuration file format that is parsed into a tree of 
  dict like Struct objects. The format supports inheritance, allowing 
  complicated configurations to be as compact as possible.

Design Goals
============
  General design/implementation goals, some have been met, others are 
  still in progress.

    - Support Twisted and non-Twisted reactor driven Python programs.
    - Scalable to complex configurations, easily avoiding duplication.
    - Orthogonal to code; code should not be required to know about the
      config system used, it should be regular Python or Twisted code.
    - Minimal boilerplate.

Text Format
===========
  Coil provides the concept of a struct - an ordered list of key/value 
  pairs. Basic types are True, False, None, integers, floats, unicode 
  strings and lists of basic types. Here we define a single struct:

    x: {
      anInt: 1
      aFloat: 2.4
      aString: "hello"
      andAList: [1 2 "hello" 4 None]
    }

  Whitespace doesn't matter, so these two are identical:

    x: {a: 1}
   
    x: {
      a: 1
    }

  Structs can extends other structs: this means they inherit all 
  attributes from that struct. Extending is done with a special 
  attribute, @extends, with a value that is a path to another struct. 
  Paths can be relative, with a prefix of ".." meaning go up one level,
  "..." go up two levels, etc., or absolute, starting from the special 
  location @root.  In this example, y and z inherit from x and override
  some of its attributes:

    x: {a: 1  b: 2}
    y: {
      @extends: ..x # relative path
      b: 3
    }
    z: {
      @extends: @root.x # absolute path
      b: 4
    }

  In this example y is the same as:

    y: {a: 1 b: 3}

  If the above example was a file called "foo.coil" and we did:

    sub: {
           @file: "foo.coil"
           x: {c : 2}
         }

  then sub.z  would be {c: 2 b: 4}.

  For extending substructs there is a shorthand syntax. In this example
  y and z both extend x, and have identical contents:

    x: { a: {b: 1} }
    y: {
       @extends: ..x
       a.b: 3
    }
    z: {
       @extends: ..x
       a: {
          @extends: ..x.a
          b: 3
       }
    }

  Structs can also be used to import files, either given a path on the 
  filesystem, which can be absolute or relative to the current coil 
  file:

    example: {@file: "/home/joe/test/example.coil"}

  or give a path which is relative to the path of Python package which 
  is present in sys.path, for example the file "example.coil" which is 
  present in the coil.test package:

    example: {@package: "ops.avs:./example.coil"}

  Links can be used to have attributes whose value is determined based 
  on their context, i.e. at lookup time rather then at parse time. 
  Their syntax is like the paths used for @extends, except that they 
  have a "=" prefixed. For example, server1.myaddress.host will be the 
  same as server1.host in this example:

    address: {host: =..host  port: 1234}
    server1: {
       myaddress: {@extends: @root.address}
       host: "www.example.com"
    }

  Finally, sub-structs can delete attributes provided by structs they 
  extend:

    base: {x: 1  y: 2}
    sub: {@extends: ..base
          ~x  # sub now has no attribute "x"
         }

  References are also allowed within strings by using ${name}. For 
  example:

    foo: "zomg"
    bar: "${foo}bbq"
    sub: { x: "foo is ${..foo}" y: "foo is ${@root.foo}" }

  expands out to be:

    foo: "zomg"
    bar: "zomgbbq"
    sub: { x: "foo is zomg" y: "foo is zomg" }

  Emacs users may be interested in the coil mode provided by 
  misc/coil.el in the coil distribution tarball.

API Overview
============
  The core of the Coil API is the Struct object. It is a dict-like 
  mapping object that knows its place in a tree and can reference items
  anywhere in the tree.

  Assume we have a file at /tmp/example.coil with the following 
  contents:

    x: { y: {a: 2}
         z: "hello"
         list: [1 2 3]}
   
    sub: {
         @extends: ..x
         y.b: 3
         ~z
    }

  We can then inspect the structure just like nested dict objects:

    >>> import coil
    >>> conf = coil.parse_file("/tmp/example.coil")
    >>> conf['x']['list']
    [1, 2, 3]
    >>> conf['x']['z']
    'hello'
    >>> conf.get('x').get('z')
    'hello'
    >>> conf.keys()
    ['x', 'sub']
    >>> 'z' in conf['sub'] # we deleted this with ~z
    False
    >>> conf['x']['y']
    Struct({'a': 2})
    >>> conf['sub']['y'] # inherited from x and added 'b'
    Struct({'a': 2, 'b': 3})

  Also, we can access and items based on absolute and relative paths as
  we can in the text format:

    >>> conf['x.z']
    'hello'
    >>> conf.get("@root.x.z")
    'hello'
    >>> x = conf['x']
    >>> x.get("..sub.y.b")
    3
    >>> conf.set("sub.y.c", 4)
    >>> conf['sub']['y']
    Struct({'a': 2, 'b': 3, 'c': 4})